The present invention relates generally to induction brazing, and, more specifically, to a method and apparatus for controlling the induction brazing process via a closed loop control system controlled by temperature sensed at the brazed joint.
Brazing is a well-known method of joining two metal pieces by fusing a layer of alloy between adjoining surfaces. It also is known to heat the brazing alloy layer in the region of the joint by use of an induction coil encircling the joined members in the plane of the brazing alloy layer (flux) for its fusion. The energy supplied to the induction coil typically is controlled by either visual operation of the heating effect on the brazing ring or by time, that is, the coil is simply powered for a predetermined time, with hopefully the desired result achieved.
This approach leads to problems, particularly when brazing is done of thin-walled members such as condenser tubes of the type used in automotive applications. Such tubes typically are brazed to the condenser via sockets and are constructed of materials with complex chemistry such as aluminum. In such cases, brazing typically is performed with an aluminum alloy. This combination of a thin-walled tube and the complex chemistry of aluminum makes for a difficult mix. For example, it is extremely difficult to control the temperature of aluminum and it is easy to melt through a thin-walled tube during the brazing process. Under heating causes inadequate penetration of the brazing ring into the brazed joint. Such heating variations are exacerbated by variations in the tubing wall dimension, variations in the amount of flux applied, variations in the air gap between the tube and the socket, and variations in the tube's position relative to the induction coil. High scrap rates have resulted from the problems inherent in such brazing processes.
For example, aluminum condensers have been induction brazed by Chrysler Corporation in a manufacturing process typically consisting of brazing 1/2 inch or 5/8 inch header tubes to a condenser socket. The tubes are manually assembled with a braze ring of an aluminum alloy prior to presentation at the induction machine. Up to 40% of the product typically either requires rework or is scrapped due to heating variations of the brazed joint.
Thus it would be desirable to provide a method and apparatus for induction brazing wherein the energy supplied to the braze alloy could be controlled in a matter to minimize defective brazed joints.
It would further be desirable to provide a method and apparatus for induction brazing wherein the power supplied to the induction coil could be controlled by a signal that is indicative of the state of the brazed joints.